1. Technical Field
The present invention relates generally to ion implanters, and more particularly, to a method and apparatus for weakening a strong focus effect of an acceleration-deceleration column of an ion implanter.
2. Related Art
Improving productivity of ion implanters that use a low energy beam is a continuing issue in the ion implanter industry. One area of focus is improving beam transport efficiency. In any ion beam, the positively charged ions tend to repel one another, which causes loss of ions and poor beam transport. Referring to FIG. 1, a conventional ion implanter 100 is illustrated. Ion implanter 100 generates and transmits an ion beam 104 to a target 106 in an implant chamber 108. Ion implanter 100 includes a gas flow 140; an ion source 142 including a source magnet 144 and a source bias voltage controller 146; a suppression electrode 148, an extraction electrode 150 and one or more manipulator motors (not shown) for electrodes 148, 150; a filter magnet 151; a filter resolving aperture 152; an acceleration-deceleration column 200; an analyzer magnet 156; a mass slit 160; a pre-scan suppression electrode 162; horizontal scan plates 164; a post-scan suppression electrode 166; a nitrogen (N2) bleed 168; a corrector magnet 170; a limiting aperture 172; and a profiler system 174. Each of the above-described components is monitored by and responsive to system controller 120. Typically, target 106 includes one or more semiconductor wafers mounted to a platen 114.
Referring to FIG. 2, ion beam acceleration-deceleration column 200 accelerates or decelerates ion beam 104 prior to mass analysis by analyzer magnet 156 (FIG. 1). Initially, ion beam 104 is formed at filter resolving aperture 202 from a ion source 142 trough extraction electrode 150 and filter magnet 151, which removes most of unselected ions from beam 104 (FIG. 1). Ion beam 104 is initially formed with high energy because it is relatively difficult to obtain a beam with high beam current at low energies from ion source 142 directly. Accordingly, a conventional ion implanter 100 is designed to run at a reasonably high source extraction voltage, for example, greater than 40 kV to give generated ions initial upstream velocity. Acceleration-deceleration column 200 applies different potential gradients that generate electric fields that decelerate (as shown in FIG. 2) or accelerate ion beam 104 depending on its mode as the ions pass through to give the ions their final velocity. Acceleration-deceleration column 200 includes terminal electrodes 204 to receive ion beam 104 into accel-decel column 200. Next, a focus electrode 206 provides an adjustable focusing effect to ion beam 104, which also produces an electron trap to prevent the neutralizing electrons from being removed from the region of the beam-line located before the deceleration region during the deceleration mode. Next, a ground electrode 208 receive ion beam 104 with its final energy. In the acceleration mode (not shown), an electric field generated between terminal electrodes 204 and ground electrode 208 forms an acceleration lens by applying a positive voltage on terminal electrodes 204 to energize and accelerate the positively charged particles of ion beam 104. Acceleration-suppression electrode 210 is biased with a negative voltage that suppresses secondary electrons during the acceleration mode. In the deceleration mode, shown in FIG. 2, an electric field generated between terminal electrodes 204 and ground electrode 208 form a deceleration lens by applying a negative voltage on terminal electrodes 204 to de-energize and decelerate the positively charged particles in ion beam 104.
As shown in FIG. 2, as acceleration-deceleration column 200 decelerates ion beam 104 and lowers its energy, it tends to drastically focus ion beam 104. In particular, as the ions leave the deceleration lens, they have a large convergent angle, i.e., they are tending to move inwardly with a large vertical velocity component. The large convergent angle causes ion beam 104 to have a large divergent angle afterwards, i.e., the ion will move outwardly with the same amount of vertical velocity component and thus increases the dispersion problem and reduce the transport efficiency of the low energy beam, i.e., ions are loss to dispersion as the beam exits accel-decel column 200. (Ion beam 104 exits via a coupling device 212, which provides a sliding vacuum seal to the analyzer magnet). The larger the energy change in ion beam 104, the more drastic the focusing and the more drastic the dispersion problem. It should be recognized that while FIG. 2 shows a two-dimensional view of ion beam 104 envelope, the focusing action is dependent on radial position within ion beam 104. That is, the closer to the inner edge of electrode 210, the stronger the focusing effect.
In view of the foregoing, there is a need in the art for improved beam transport efficiency relative to an acceleration-deceleration column of an ion implanter.